The present invention is concerned, in general, with improvements to blast cleaning apparatus and, in particular, with improvements to a media valve and a metering and dispensing system used to control by means of differential pressure the amount of abrasive media directed into a compressed air stream.
Standard sand blasting equipment consists of a pressure vessel or supply pot to hold particles of a blasting medium such as sand, a source of compressed air connected to the supply pot via a conveying hose and a means of metering the blasting medium from the supply pot, which operates at a pressure that is the same or slightly higher than the conveying hose pressure. The sand/compressed air mixture is transported to a nozzle where the sand particles are accelerated and directed toward a workpiece. Flow rates of the sand or other blast media are determined by the size of the equipment. Commercially available sand blasting apparatus typically employ media flow rates of 10-20 pounds per minute. About 0.5 to 1 pound of sand are used typically with about 1.0 pound of air, thus yielding a ratio of 0.5 to 1.0.
When it is required to remove coatings such as paint or to clean relatively soft surfaces such as aluminum, magnesium, plastic composites and the like, or to avoid surface alteration of even hard materials such as stainless steel, less aggressive abrasives, including inorganic salts such used in place of sand in conventional sand blasting equipment. The media flow rate used for the less aggressive abrasives is substantially less than that used for sand, and has been determined to be from about 0.5 to about 10.0 pounds per minute, using similar equipment. The lower flow rates require a much lower media to air ratio, in the range of about 0.05 to 0.5.
However, difficulties are encountered in maintaining continuous flow at these low flow rates when conventional sand blasting equipment is employed. The fine particles of an abrasive media such as sodium bicarbonate are difficult to convey by pneumatic systems by their very nature. Further, the bicarbonate media particles tend to agglomerate upon exposure to a moisture-containing atmosphere, as is typical of the compressed air used in sand blasting. Flow aids such as hydrophobic silica have been added to the bicarbonate in an effort to improve the flow, but maintaining a substantially uniform flow of bicarbonate material to the blast nozzle has been difficult to achieve. Non-uniform flow of the blast media leads to erratic performance, which in turn results in increased cleaning time and even to damage of somewhat delicate surfaces.
Commonly assigned U.S. Pat. Nos. 5,081,799 and 5,083,402 disclose a modification of conventional blasting apparatus by providing a separate source of line air to the supply pot through a pressure regulator to provide a greater pressure in the supply pot than is provided to the conveying hose. This differential pressure is maintained by an orifice having a predetermined area and situated between the supply pot and the conveying hose. The orifice provides an exit for the blast media and a relatively small quantity of air from the supply pot to the conveying hose, and ultimately to the nozzle and finally the workpiece. The differential air pressure, typically operating between 1.0 and 5.0 psi with an orifice having an appropriate area, yields acceptable media flow rates in a controlled manner. The entire contents of U.S. Pat. Nos. 5,081,799 and 5,083,402 are herein incorporated by reference.
A media metering and dispensing valve which meters and dispenses the abrasive from the supply pot through the orifice and to the conveying hose carrying the compressed air stream typically operates automatically whenever the compressed air is applied to the blast hose to begin the abrasive blasting operation. The media valve for use in the afore-mentioned metering and dispensing process as disclosed in U.S. Pat. Nos. 5,081,799 and 5,083,402 is characterized as a Thompson valve and is described in detail in U.S. Pat. No. 3,476,440, the contents of which are herein incorporated by reference. The Thompson valve includes a metering valve stem which blocks the outlet of a discharge tube disposed between the supply pot and an air flow tube which is secured to and carries the compressed air to the conveying hose. When the blast nozzle is activated, the valve stem is lifted from the valve seat of the Thompson valve and allows a controlled amount of media to flow through the outlet of the discharge tube into the air flow tube. The valve as disclosed in U.S. Pat. No. 3,476,440 has been improved by placing the valve stem within a control sleeve which contains a plurality of orifices having different sizes, one of which can be placed in communication with the outlet of the discharge tube and the air flow tube. When the valve stem is seated within the valve body and closed, the orifice in the control sleeve is blocked such that media cannot flow from the discharge tube through the orifice in the media control sleeve and then into the air flow tube. Upon operation of the blast nozzle, the valve stem is pulled away from the orifice to allow the media flow from the pot to the discharge tube, through the orifice and into the air flow tube.
The plurality of orifices provides another means of controlling the amount of media flowing from the supply pot into the compressed air stream and into the blast nozzle apparatus. Unfortunately, to change the orifice which is in alignment with the media discharge tube and the air flow tube or to clean out a plugged orifice in the Thompson valves now on the commercial market, it is required that the valve body holding the stem be taken apart, the valve sleeve taken out, rotated, placed back in its slot and the valve body then restructured. Obviously such disassembly and reassembly is cumbersome and certainly does not allow for efficient blast cleaning on the job site.
The present assignee has developed a novel and improved media control valve which is particularly useful in the differential pressure metering system of U.S. Pat. Nos. 5,081,799 and 5,083,402. The improved metering valve offers additional control with respect to metering the flow of media. Like the prior art Thompson media control valves, the novel valve includes a control sleeve which contains a plurality of orifices, one of which can be aligned to communicate with the discharge of the media from the supply pot and the air flow tube to dispense the media into the compressed air stream. The plurality of orifices have a different diameter to allow enhanced control of the amount of media dispensed from the supply pot to the compressed air flow tube by allowing a change of orifice size. Importantly, to control the metering of the abrasive media into the air flow tube, the control sleeve can be rotated while in place in the valve body to align a different orifice with the media discharge passage in communication with the supply pot and the compressed air flow tube. Alternative embodiments are provided to index the control sleeve such that an orifice is properly aligned upon rotation of the control sleeve. In one embodiment, the index means comprises a ball spring plunger placed in the valve body and exerted against the control sleeve and a series of detents spaced in the sleeve and aligned with each orifice so as to properly align the orifice with the media discharge passage from the supply pot and the air flow tube when the ball spring plunger fits within a detent in the sleeve. The control sleeve which contains the valve stem can be easily removed from the valve body in one piece for cleaning and replaced and locked in place in the valve body by means of a lock pin without disassembling the body of the valve. In the second embodiment, the index means comprises a plurality of grooves which are placed on the face of the bore which receives the control sleeve and which mate with a plurality of teeth on the control sleeve. The teeth are aligned with the orifices. To change orifices, the control sleeve is lifted to disengage the teeth from the grooves and rotated until the teeth and grooves are again aligned and the sleeve then dropped in place in the valve body. The media control valve also includes a manually adjustable multi-port valve placed within the media discharge passage and which can close off the discharge passage from the supply pot, and allow compressed air to back clean the valve and direct debris out a clean-out port in the valve body. The novel and improved media control valve is described in commonly assigned, copending application, U.S. Ser. No. 161,530, filed Dec. 6, 1993, the entire contents of which are herein incorporated by reference.
Although the improved media valve as described in the above commonly assigned, copending application can be used with any system which meters the abrasive media from the supply pot to the compressed air line, the improved valve has particular use in the differential pressure supply system as described in above-mentioned commonly assigned U.S. Pat. Nos. 5,081,799 and 5,083,402. While the media flow rates achieved utilizing these patented control systems have been found acceptable, it is has been found that improvements can be made. In particular, it has been found that the media flow rates can sometimes be inconsistent during the blast cleaning operation. It is believed that these inconsistencies are due to the presence of turbulent air below the orifice which causes the media to "see" a slightly different air pressure than what is actually present in the compressed air line into which the media is ultimately dispensed. The turbulence is believed to be caused by the air passing across the outlet of the media discharge tube which carries the abrasive media from the orifice into the air flow tube. The air turbulence at the discharge tube outlet can vary the air pressure immediately below the orifice. Since the differential air pressure between the supply pot and the air line which is calculated and used to meter the abrasive media operates only between 1.0 and 5.0 psi, fluctuations in the air pressure immediately below the orifice can result in an inaccurate real time differential pressure and consequent inconsistent media flow rates through the orifice and variations in the ultimate media concentration in the compressed air stream. In particular with sodium bicarbonate media in which lower amounts of media are utilized per amount of compressed air, small changes in the concentration of the media in the compressed air can result in pronounced uneven blast cleaning.
Sodium bicarbonate media is lighter than the sand abrasive and accordingly, the bicarbonate media can be more easily manipulated by control of air pressure. Thus, minor changes in the apparatus used to control media flow from the hopper to the air line are very useful in manipulating the flow of the bicarbonate media. While the improved media valve utilized to control the media flow is useful with any abrasive, it has been found particularly useful with the lighter bicarbonate media inasmuch as controlling the flow rate of a sand abrasive cannot be readily achieved by small changes in the metering apparatus due to the heavier density and mass of the sand abrasive particle. Flow rate control of sand relies typically on gravity flow alone.
Another cause of turbulent air is found in the orientation of the compressed air line itself. Typically, due to the confines of space, the compressed air line from the compressed air supply source often bends immediately before passing across the outlet of the discharge tube from the media control valve. The air flow immediately past the bend is often turbulent, again, resulting in inconsistencies between the calculated and real time differential pressure between the media supply hopper and the compressed air line immediately below the orifice of the valve.
Accordingly, it is an object of the present invention to provide improvements in the metering and dispensing apparatus described in U.S. Pat. Nos. 5,081,799 and 5,083,402.
It is another object of the present invention to provide an improved metering and dispensing system for metering an abrasive into a compressed air line by means of differential pressure between the supply of abrasive media and the compressed air line.
Still another object of the present invention is to provide for more uniform metering and dispensing of an abrasive into a compressed air line using a differential air pressure metering system and a Thompson media control valve wherein the abrasive media is passed through an orifice and into the compressed air line.
Yet another object of the invention is to reduce turbulent air flow through the compressed air line at the point of contact with the media outlet from the metering valve.